1. Field of the Invention
This invention is related to nanoscale switching devices in which conduction through the switching devices is controlled by quantum effects.
2. Description of the Related Art
From the vacuum tube to the modern CMOS transistor, devices which control the flow of electrical current by modulating an electron energy barrier are ubiquitous in electronics. In this paradigm, switching the electrical current by raising and lowering the barrier, which must have a height greater than kBT, generates a commensurate amount of heat, thereby necessitating incredible power dissipation at device densities approaching the atomic limit.
Since about 1960, the steady downscaling of integrated circuit minimum dimensions has permitted ever-increasing density and thus an ever-increasing range of functionality at an ever-more favorable cost. This capability has permitted system designers to introduce many of the electronic products which have revolutionized industry and daily life in these decades. Continued downscaling steadily improves the available functionalities and pricing, while steadily challenging system designers.
However, it is expected that the downscaling of minimum geometries of transistor-based integrated circuits will eventually be brought to an end by a combination of problems related to devices, interconnections, noise, and reliability. These problems include power dissipation, limitations in lithographic printing, thermal stability, dopant diffusion lengths, punch-through, doping levels, electric fields, and hot electrons to name a few.
Conventional semiconductor integrated circuit technology uses a monolithic substrate which is all one crystal. Such substrates provide great advantages in processing. However, such device architecture poses difficulties for future scaling. One difficulty is lateral isolation of devices from each other. Another difficulty is leakage current scaling. Another difficulty is the diffusivity of carriers within the substrate, as free carriers (generated, e.g., by an alpha particle hit) can diffuse over many tens of microns and neutralize a stored charge.
Nevertheless, progress in semiconductor nanofabrication and nanoscale spatial and charge quantization phenomena has bridged the gap from the 0.1 micron regime to the 10's of nanometer scale, and even to the atomic level with scanning probe techniques. These advances allow one to create electronic structures that exhibit manifest quantum and single-electron effects.
However, many proposed solid state device implementations at this level suffer from a number of problems. For example, critical dimensional control for devices that operate by tunneling is a problem since a barrier (e.g., a heterostructure, oxide, or otherwise) is a prerequisite for isolation in a 3-terminal device that can exhibit gain, and dimensional variations result in decidedly different device performance. Moreover, electron tunneling is exponentially sensitive to atomic-layer fluctuations in the tunneling barriers, resulting in device characteristic variations unacceptable for large scale integration. Meanwhile, device embodiments utilizing discrete electron charging (single-electron transistors, or SETs) suffer from reduced operating temperatures; for room temperature operation, 1 nm or less size junctions are required, dimensions which imply severe tunnel barrier fluctuation problems for solid state embodiments.
Another possible alternative to traditional electron current modulation is to exploit the wave nature of the electron to control current flow on the nanoscale. In traditional mesoscopic devices, interference of electron waves is typically tuned via the Aharanov-Bohm effect. However, for nanoscale devices such as single molecules, this is impractical due to the enormous magnetic fields required to produce a phase shift of order 1 radian. Similarly, a device based on an electrostatic phase shift would require, for the small molecules being considered, voltages incompatible with structural stability.
Thus, prior work directed to miniaturizing switching devices to the molecular dimension has been frustrated by a suitable means to control and/or modulate current flow through the nano-sized medium.